A blocking of an ophthalmic lens blank is required prior to working the lens blank in a lens generating apparatus. Modern ophthalmic lens generating apparatus of particular interest herein are computer-controlled machines capable of generating a final optical surface on an ophthalmic lens in a single operation. These apparatus also include profiling machines capable of shaping the contour of the lens while the lens blank remains bonded to the original support block used during the surface generation process.
As can be appreciated, the precision of these machines depends greatly upon the accuracy with which the lens blank is affixed to the support block. As may also be appreciated, the efficiency of these machines depends greatly upon the available information about the lens blank, for programming the controller of the machine. For example, when the locations and curvatures of both surfaces of a lens blank are known precisely, it is possible for the controller of the machine to calculate the amount of material to be removed during the surface generating process, the exact coordinates for an initial cut, the total number of cuts required, the depth of cut for a finishing pass and the total duration of the surface generating process. These values may thereafter be analysed for devising an optimum cycle time for manufacturing an ophthalmic lens, and for generating lens of a highest quality.
Generally, it has been difficult with the lens blockers of the prior art, to obtain accurate information about the surfaces and thickness of a lens blank for supporting a significant optimization of the efficiency of a lens generating apparatus. In the past, emphasis has been placed on the precision in the positioning of the optical references of a lens blank relative to the support block. The physical locations and curvatures of both surfaces have been of a secondary interest.
For this reason also, and despite the use of ultra-precision computer-controlled apparatus, ophthalmic lens generated nowadays are not always errorless. Ophthalmic lens are often thicker than they could be for providing a good appearance when mounted in a thin frame for example. Another common error found in ophthalmic lenses is referred to in the industry as spherical aberrations in the lenses. This type of error is related to the inconstancy of the focal length of the incident rays of light passing through the lens at various locations across the surface of the lens and especially near the edges of the lens. Both types of errors are often related, amongst other things, to an insufficient knowledge of the exact location and curvature of at least one surface of the lens blank.
The machines of the prior art for blocking a lens have evolved from a basic manual and visual alignment of markings on the lens blank with references on the support block, to semi-automatic equipment using a pick and place manipulator and LCD generated target images for positioning the lens blank. In this respect, a representative group of lens blockers of the prior art is presented hereinbelow.
In a first example of lens blocker of the prior art, the U.S. Pat. No. 3,804,153 issued on Apr. 16, 1974 to Luc Andre Tagnon illustrates a device for positioning a mold for casting a metal block onto the surface of a lens. The mold is movable by means of a spherical swivel joint and pantograph linkage such that the reference axes of the mold are movable to coincide with the optical axes of the lens blank, and such that the mold always makes direct contact with the curved surface of the lens. The reference axes on the metal block are later used for controlling the movements of a lens trimming and bevelling machine.
A second example of lens blocking device of the prior art is described in the U.S. Pat. No. 4,288,946 issued on Sep. 15, 1981 to Bela J. Bicskei. The device comprises a tripod on which an ophthalmic lens is placed, and a projector and minor assembly for projecting a target image from under the tripod. The lens is manually positioned on the tripod with the optical markings on the lens corresponding to the target image of the projection. When the lens is properly positioned, an articulated blocking aim is used for precisely positioning and securing a support block to the lens blank.
In another example, the U.S. Pat. No. 4,319,846 issued on Mar. 16, 1982 to David W. Henry et al. describes a method and apparatus for aligning a lens blank upon a lens blocking station. A transparent indicia is movable back and forth over a lens supported on a blocking station. The indicia is firstly used to properly position the lens blank over the blocking station, with the optical axes of the lens blank being in a consistent orientation relative to the blocking station.
Another example of lens blocking devices of the prior art is described in U.S. Pat. No. 5,283,980 issued on Feb. 8, 1994 to Marold H. Lohrenz et al. With this device, a lens blank is placed over a sheet of non-slip transparent material covering a liquid crystal display connected to a computer. The LCD exhibits a target image generated by the computer. The lens is manually positioned over the sheet of non-slip material according to the markings of the target image. The computer calculates and compensates for the optical error in the refractive characteristics of the non-slip sheet and the viewing glass, and shifts the target image to account for the error. Once the lens is properly positioned, a lens support block is affixed to the lens blank using a blocking arm capable of applying a constant force to the lens independently of the height of the lens blank.
The U.S. Pat. No. 5,498,200 issued on Mar. 12, 1996 to Ralf Werner describes a device for attaching a holder to a lens blank before grinding the edge of a lens blank. The lens blank is placeable over a glass plate carrying a tripod stand. A LCD screen is used to project a scale, a template image or an eyeglass frame opening image. A prism is positioned between the lens blank and the eye of an operator of the device for superimposing the projection of the LCD screen onto the image of the lens blank. The tripod is raiseable to a predetermined height before the lens blank is adjusted on the tripod and a support block is affixed thereto.
A last example of a lens blocker of the prior art is described in U.S. Pat. No. 5,505,654 issued on Apr. 9, 1996 to Kenneth O. Wood et al. The apparatus comprises an alignment station for supporting and aligning the lens blank with a target image generated by a liquid crystal display. Both the image of the lens and the target image are projected on a viewing mirror in front of the operator of the apparatus. The apparatus also has a movable pick and place arm with a vacuum picking cup for moving the lens from the alignment station to a blocking station while maintaining the lens orientation. The blocking station includes a support for a lens block, a support for the lens blank and a system for injecting heated liquid bonding material between the lens and the block which solidifies on cooling to join the lens and block together.
As explained before, the devices and apparatus of the prior art have attached little importance to the measurement of the exact positions of both surfaces of a lens blank, and to the verification of the curvature of these surfaces. As a result, a first surface is often generated in a lens blank without knowing precisely where the second surface really is.
Furthermore, some devices and apparatus of the prior art use lens supports and vacuum picking cups made of resilient material such as rubber for example. Although the positioning of the lens is precisely effected at the imaging station, the manipulation of the lens using flexible members prior to bonding the lens can cause slight misalignment of the lens during the bonding of the lens to the support block thus causing a prism error in the lens.